feat(solana): remove txs field in relayer, encode txs, parallel relayer packets submission

[vaporif]

Description

closes: #XXXX

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• Linked to GitHub issue with discussion and accepted design, OR link to spec that describes this work.
• Wrote unit and integration tests.
• Added relevant natspec and godoc comments.
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@              Coverage Diff               @@
##             feat/solana     #771   +/-   ##
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  Coverage               ?   99.86%           
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  Files                  ?       17           
  Lines                  ?      764           
  Branches               ?        0           
==============================================
  Hits                   ?      763           
  Misses                 ?        1           
  Partials               ?        0           

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